1. Technical Field
This invention relates generally to flexible circuit substrates, and more specifically to a tear resistant flexible circuit substrate.
2. Background Art
Over one hundred and forty million Americans now use a cellular telephone and another fifteen million or so are expected to subscribe in the coming year. Sales of cellular phones have risen faster than those of facsimile machines, subscriptions to cable television service, and sales of videocassette recorders. Cellular telephones have become a feature across both the business and recreational landscapes. Customers have come to expect, and demand, steady increases in reliability and portability of the telephones. They have also come to demand the constant reductions in cost of the telephones.
A critical aspect of the reliability of the cellular telephone is the reliability of its power source, the battery. The battery may well be the single most important feature in a cellular telephone, or for that matter, in other types of electronic devices, including two-way radios. Consumer surveys have shown that talk time is the feature valued by users of cellular telephones and two-way radios above all else.
At the same time, consumers are demanding smaller and smaller phones. Consequently, the non-cell components of the corresponding battery pack, like the safety circuitry, charging circuitry and fuel gauging circuitry, have become smaller to deliver greater talk time without increasing the size of the battery. The advent of flexible circuits has been integral in the size reduction of these circuits.
By way of background, not too long ago, electronic circuits were mounted upon printed circuit boards. These printed circuit boards were rigid, flat boards made of layers of fiberglass with copper pads and traces disposed atop and between these layers. Printed circuit boards were difficult to work with in the small confines of a batter pack in that they were both bulky and rigid. The thickness of the printed circuit boards increased the overall thickness of the pack, and the rigidity prevented the printed circuit board from folding about the cell.
To remedy these issues, a new substrate, known as a “flexible circuit substrate” was developed. These substrates, affectionately known as “flexes”, are generally manufactured from polyimide films, like Kapton®, manufactured by the DuPont Company (See, e.g., www.dupont.com/kapton/). Kapton® is a semi-transparent film that is durable, flexible and heat resistant, and is used in applications ranging from circuit substrates to automotive wiring harnesses to solar cell and space exploration applications.
By depositing conductive copper pads and traces atop and between layers of Kapton®, durable flexible circuits are made. The use of flexible circuits in battery packs is well known. For example, commonly assigned U.S. Pat. No. 6,153,834, entitled “Flexible Circuit with Tabs for Interconnection to Battery Cells, issued Nov. 28, 2000, incorporated by reference herein, teaches the use of a flexible circuit in conjunction with a battery pack.
One problem that exists with polyimide films like Kapton® involves tearing. Much like a bag of potato chips that is initially difficult to open, but once torn tears very easily, polyimide films tend to tear very easily once a tear has been initiated. As many battery pack designs, like that taught in the '834 patent mentioned above, employ flexes that have extensions that are bent in different directions, any tearing of the flex may render the battery inoperable. The reason for this is that tears in the flex may in turn tear critical conductive traces within the flex.
There is thus a need for an improved flexible circuit substrate that is resistant to tearing.